1. Field of the Invention
The present invention relates to measurement of temperatures inside a vacuum chamber and, more particularly, to apparatus and methods for accurate, real time measurement and control of semiconductor wafer temperatures during processing in a vacuum processing chamber.
2. Prior Art Technology and Problems
At least several factors require fast, accurate, real time measurement of semiconductor wafer temperatures during plasma processing such as the etching or stripping of photoresist from semiconductor wafers. First, such processes are strongly temperature dependent. In fact, such processes may be an exponential or near-exponential function of temperature. Second, to achieve process control, frequently it is necessary to quickly ramp up the wafer temperature from ambient or near-ambient to the elevated processing temperature, and/or, after processing, quickly ramp down the temperature. Third, PID (proportional, integral, differential) and other automated temperature monitoring techniques are, or are fast becoming, the industry choice for precise temperature control during semiconductor wafer processing. Individually and collectively, these and other factors require fast, accurate, real time monitoring and/or control of semiconductor wafer temperatures during in vacuo processing (etching and deposition). Unfortunately, fast accurate temperature monitoring under vacuum processing conditions is inherently difficult and may be made more difficult by process conditions such as very low processing pressures (high vacuum) and low temperatures.
Indirect wafer temperature monitoring techniques, such as infrared and pyrometer techniques are available to the semiconductor processing industry. Typically, however, such techniques are insufficiently precise at lower temperatures. For example, accurate low temperature monitoring using infrared techniques is hampered by the relatively low intensity of radiation emanating from silicon semiconductor wafers below about 300.degree. C. Furthermore, the variation in emissivity from wafer to wafer makes radiation-based techniques unreliable for use in a process chamber which sees a wide variety of wafer types (for example, different dopant levels). Also, the presence of heating lamps may present a difficulty with respect to noise.
Direct temperature measurement, that is, monitoring the wafer temperature at the wafer using thermocouples or other temperature sensors, also has been used. However, factors such as the degradation of the sensor by corrosive processing environments, interaction between the sensor and the heating medium, and the difficulty in maintaining a stable thermal connection between the sensor and the wafer typically preclude achieving the requisite monitoring accuracy and sensor longevity.